The invention is directed to a femoral portion of a hip joint prosthesis.
As is known, femoral shank prostheses are used to replace the inoperable proximal femur, particularly the femur head. During implantation the femur head and femur neck are surgically removed, and the prosthesis shank is driven into the free-drilled femoral canal. The shank is either secured in the femoral canal by bone cement or retained solely by a positive connection (cementless prosthesis). In the latter case the prosthesis shank is relatively elongated and provided with surface irregularities, holes or the like in order to allow an ingrowth of the bone.
Two complications may occur in connection with the femoral portions of the hip joint prosthesis which cannot be completely mastered. The load in the proximal portion of a femur is a tension-thrust-load. In the lateral portion above all a tension load occurs while in the medial portion a thrust-load is prevailing. Since the shank prosthesis is approximated the shape of the proximal femur the prosthesis is loaded in a similar manner. Endoprostheses normally consist of a metal or a metal alloy compatible to the body. The materials used for this purpose have an elasticity modul deviating from that of the natural bone which by the way has a very different interior structure in dependence from the load applied to. The normally occurring load is a so-called alternating load which causes continuously a relatively small work of deformation in the bone or the prosthesis, respectively. This work of deformation causes a loosening of the prosthesis above all in the transient range between the prosthesis and the adjacent bone surface whereby the hip joint prosthesis becomes unoperable and occasionally must be removed. A loosening is occurring also if bone cement is used. The bone cement is fixedly connected with the bone while the loosening occurs between the cement and the prosthesis or between the cement and the bone, respectively, or between both. The re-implantation of hip joint prostheses is a very complicated procedure which heavily stresses the patient.
If the shank prostheses are not forged or made of unsuitable alloy material this leads to fractures from time to time. Sometimes such breakages or fractures rely on prosthesis portions not perfectly cast, the prosthesis portions may contain tension cracks or shrinking holes or the like. This phenomenon, however, is relatively seldom. Rather, it has been stated that also completely operable prosthesis shanks may fracture if they are implanted over a longer time period. In these cases so-called fatigue fractures happen.